Small Scale Processing of Coffee, Part 1 Dry Processing

The two most important species of coffee are Coffea arabica (Arabica coffee) (over 70% of world production) – and Coffea canephora (Robusta coffee). Two other species which are grown on a much smaller scale are Coffea liberica (Liberica coffee) and Coffea dewevrei (Excelsa coffee).

Arabica (Coffea arabica). This is a shrub or small tree with relatively small glossy leaves and small fragrant white flowers. Arabica coffee usually receives a premium for its superior flavor and aroma. Arabica is more suited to higher cooler climates (600-2000m altitude and 15-20°C).

Robusta (Coffea canephora). There are many different Robusta varieties. In general, they can thrive in hotter lowland areas (below 900m altitude and over 20°C). Robusta coffee is preferred for instant coffee production since the yield of soluble solids is relatively high.

Liberica (Coffea liberica). This is a larger tree with large leaves and berries. It can tolerate hot and wet conditions. Liberica coffee is grown in Malaysia and in West Africa, but only very small quantities are traded as demand for its flavor characteristics is low.

Harvesting

a. Selecting seeds. Good coffee cannot be made from poorly harvested coffee cherries. Only large and fully ripe berries from disease-free, pest-free and high-yielding trees should be selected. The coffee cherries should be picked when they are bright red all over. At this stage, the bean can be squeezed out from the pulp by applying light pressure between finger and thumb. Small shrivelled, lightweight and abnormal berries or dry, over-ripe berries should not be used.

b. Processing. There are two ways coffee can be processed – dry (‘natural’) processing and wet (‘fermented and washed’) processing. Wet processing is regarded as producing a higher quality product. The dry process, also known as unwashed or natural coffee, is the oldest method of processing coffee but it is now limited to regions where water or infrastructure for machinery is scarce.

Dry processing

a. Drying

The entire cherry after harvest can be placed in the sun for 10-14 days to dry on tables or in thin layers on concrete drying areas. Once the skin is dry, the pulp and parchment are removed from the bean. The bed depth should be around 3 cm and the cherries should be raked frequently to prevent mould-growth, fermentation or discoloration. However, contamination by dust and dirt blown onto the produce is a problem, and rain can soak the produce very quickly.

b. Solar drying

Figures 1 and 2 are designs for two solar driers – the solar cabinet drier and the Exell solar drier. The coffee should be placed in the trays in the solar drier no deeper than 3 cm and it is better if the whole tray area is covered.

Figure 1 : The solar cabinet drier The drier should be filled with coffee as early in the day as possible so that all possible sunlight hours are used. The coffee should be stirred regularly so that a uniform coloration is formed. At night, the crop should be transferred to a cool dry room to prevent moisture condensing on the coffee.

c. Artificial driers

In the wet season solar drying of produce is difficult and alternative driers are necessary.

Hulling. The dried cherry is then hulled to remove the pericarp. This can be done by hand using a pestle and mortar or in a mechanical huller. The mechanical hullers usually consist of a steel screw, the pitch of which increases (and therefore the pressure) as it approaches the outlet so removing the pericarp.

Cleaning. The hulled coffee is cleaned by winnowing.

Wet processing. In this method the cherry is squeezed in a pulping machine or pestle and mortar which removes the outer fleshy material (mesocarp and exocarp) leaving a bean covered in mucilage. This mucilage is fermented and dispersed. The bean is then washed and dried.

Pulping. This involves the removal of the outer red skin (exocarp) and the white fleshy pulp (mesocarp) and the separation of the beans from the pulp. Immature cherries are hard and green and very difficult to pulp. If the coffee is to be wet-processed, correct harvesting is essential. The two most common pulpers most suitable for small-scale units are the drum and the disc pulpers.

d. Drum Pulpers (Figure 3)

This pulper comprises a rotating drum with a punched sheet surface and adjustable breast plate between which the coffee cherries are pulped. The pulp and the beans are then separated using a simple gravity system. The distance between the drum and the breast plate has to be adjusted so that the pulp is removed without the beans being damaged.

These can be manually operated but larger scale units are normally fitted with a motor.